The increase in consumer demand for applications such as high definition television, streaming videos, video telephony and other high bandwidth and high speed applications have driven communications to higher level of technology development where service providers are looking towards the field of fiber optics for the next generation of communications. In particular, Passive Optical Networks (PON) have grown in consideration for the implementation of Fiber To The Home (FTTH) networks due to its cost effectiveness in comparison to other existing alternatives resulting from reduced fiber and electronic equipment requirements compared to the existing alternatives.
In general, PON is a point-to-multipoint network architecture consisting of an Optical Line Terminal (OLT) and a plurality of Optical Network Units (ONU), in which data is transmitted upstream and downstream from the ONU to OLT. However, since upstream transmission is multipoint-to-point, ONUs are unable to transmit the data continuously; hence the need for burst mode—where the upstream channel is shared in Time Division Multiplexing (TDM) mode and each ONU is allocated a time slot for the transmission of an optical packet.
As the distance between each ONU and OLT can vary greatly, so does the range of optical signal power levels received by the OLT, with power levels in sequential burst mode packets varying up to ratios of 1000:1 and above. This provides a problem for burst mode receivers, where there is a need to process the signals of vastly differing amplitudes and phases into packets of similar amplitudes and phases.
Conventional burst mode receivers such as that disclosed in U.S. Pat. No. 6,909,082 utilize a DC coupled single-ended solution, with a single trans-impedance amplifier output as opposed to differential outputs. One such burst mode receiver utilizes a fast Automatic Threshold Controller (ATC) mechanism to adjust the gain on an optical signal via means of an Automatic Gain Control (AGC) and sets the threshold accordingly. However, since this single-ended solution only utilizes one of the trans-impedance amplifier's differential outputs in setting up the connection, a sensitivity penalty is suffered as only half of the information is used. There is also a compromise in the symmetry of the input at the post amplifier causing asymmetry output of the same that results in a severe sensitivity penalty.
Other prior art burst mode receivers such as that disclosed in U.S. Pat. No. 6,420,928 use a differential DC connection which AC couples the input signals of the trans-impedance amplifier. The use of a capacitor at the input stage of the trans-impedance amplifier results in loss of burst mode data due to the charging and discharging times of the capacitor. This is due to the fact that the need for a short time constant in order to recover DC levels as quickly as required contradicts the need for a long time constant during idle periods where a stable DC level is required.